1. Field of the Invention
The present invention relates generally to an image sensor and a process thereof, and more specifically to an image sensor and a process thereof that has an anti-reflective layer having different thicknesses corresponding to each color filters.
2. Description of the Prior Art
CMOS image sensors (CIS) are based on CMOS technologies, so the CMOS image sensors are compatible with typical CMOS fabrication processes. They are an integration of additional signal processing logic elements on the same substrate, wherein disposing a sensor array is permitted. Thus, CMOS image sensors (CIS) are widely used.
The CIS system traditionally uses the Front Side Illumination (FSI) technology to form pixels of a pixel array. In FSI CMOS image sensors, light is transmitted to a photo-sensing area through the front side of the pixel. This means that the incident light has to first pass through dielectric layers, and metal layers before it reaches the photo-sensing area, thereby causing a low quantum efficiency (QE), serious cross talks between pixels, and dark current.
Another type of CMOS image sensor is the BSI (Back Side illumination) CMOS type of image sensors. Instead of illuminating a CMOS image sensor from the top (front) side of the silicon die, a BSI CMOS image sensor uses color filters and microlenses on the back side of the pixels so that the incident light is collected from the back side of the image sensor. Compared to the FSI CMOS image sensors, the BSI CMOS image sensors have lower light losses, reduced cross-talks, and better quantum efficiency.
A conventional back side illumination (BSI) image sensor may be divided by function into a light sensing area and a peripheral electronic circuit area. The light sensing area has a plurality of photodiodes arranged in an array, and MOS transistors to sense light intensity, i.e. a reset transistor, a current source follower and a row selector. The peripheral electronic circuit area connects interconnects to external connections. A main function of the back side illumination (BSI) image sensor is to divide incident beams into combinations of light of different wavelengths. The light is received by a plurality of imaging devices on the semiconductor substrate and transformed into digital signals of different intensities. For instance, an incident beam is divided into a combination of red, green and blue light and received by corresponding photodiodes. Each photodiode transforms the light intensity into digital signals.